1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of the Related Art
As described in, for example, Japanese Patent Application Publication No. 2010-155598 (JP 2010-155598 A), an electric power steering system (hereinafter, referred to as “EPS”) assists a driver's steering operation by applying torque generated by a motor to a steering mechanism of a vehicle. The EPS executes feedback control of motor currents to generate an appropriate assist force based on at least a steering torque. That is, the EPS adjusts the voltage to be applied to the motor by adjusting the duty ratio through pulse width modulation (PWM) such that the difference between an assist current command value, computed on the basis of at least the steering torque, and a detected motor current value is decreased.
Because the EPS is required to ensure a higher level of safety, the EPS described in JP 2010-155598 A employs the following configuration. That is, the EPS limits the assist current command value to a value within a range between prescribed upper and lower limits, inclusive, when the sign (positive sign or negative sign) of the steering torque and the sign (positive sign or negative sign) of the assist current command value coincide with each other. On the other hand, when the steering torque and the assist current command value are opposite in sign to each other, the EPS determines that an abnormality has occurred in the assist control computation and limits the assist current command value to zero.
However, the EPS described in JP 2010-155598 A leaves room for improvement in the following respect. That is, the EPS described in JP 2010-155598 A is not able to limit the assist current command value to zero when the steering torque is within a range of small values (a range centered at zero and between prescribed positive and negative values, inclusive). Generally, the assist current command value is generated by adding a compensation amount for adjusting the steering behavior to a basic component based on the steering torque. However, the sign of the compensation amount does not coincide with the sign of the steering torque in some cases. When the steering torque is large, even if the sign of the compensation amount does not coincide with the sign of the steering torque, the compensation amount is cancelled out by the basic component and thus the sign of the assist current command value itself coincides with the sign of the steering torque. Therefore, the non coincidence between the sign of the assist current command value and the sign of the steering torque may be regarded as occurrence of an abnormality of the assist control computation. However, when the steering torque is within the range of small values, the basic component is small and the ratio of the compensation amount to the assist current command value is high. Therefore, even if there is no abnormality in the assist control computation, there is a possibility that the sign of the assist current command value and the sign of the steering torque will not coincide with each other. In this case, if the assist current command value is limited to zero, there is a possibility that the steering behavior will not be adjusted.
Therefore, when the steering torque is within the range of small values, the EPS described in JP 2010-155598 A does not limit the assist current command value to zero, but limits the assist current command value within a range having a margin where the compensation amount is not limited. Therefore, even if an abnormal assist current command value is erroneously computed for some reason, the assist current command value is not strictly limited when the steering torque is within the range of small values. Thus, an unintended assist force may be applied to the steering mechanism and self-steer may occur in some cases.